Secondary cells using rechargeable hydrogen storage negative electrodes are an environmentally non-threatening, high energy density electrochemical power source. These cells operate in a different manner than lead acid, nickel-cadmium or other battery systems. The rechargeable hydrogen storage electrochemical cell or battery utilizes a negative electrode that is capable of reversibly electrochemically storing hydrogen. These cells usually employ a positive electrode of nickel hydroxide material, although other positive materials may be used. The negative and positive electrodes are spaced apart in an alkaline electrolyte, which may include a suitable separator, i.e., a membrane, therebetween.
Upon application of an electrical potential across the cell, the negative electrode material (M) is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of hydroxyl ion: EQU M+H.sub.2 O+.sup.- M--H+OH.sup.- (Charging)
Upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron: EQU M--H+OH.sup.- M+H.sub.2 O+e.sup.- (Discharging)
In the reversible (secondary) cells of the invention, the reactions are reversible.
The reactions that take place at the positive electrode of a secondary cell are also reversible. For example, the reactions at a conventional nickel hydroxide positive electrode as utilized in a hydrogen rechargeable secondary cell are: EQU Ni(OH).sub.2 +OH.sup.- NiOOH+H.sub.2 O+e.sup.- (Charging), EQU NiOOH+H.sub.2 O+e.sup.- Ni(OH).sub.2 +OH.sup.- (Discharging).
A secondary cell utilizing an electrochemically rechargeable hydrogen storage negative electrode offers important advantages over conventional secondary cells and batteries, e.g., nickel-cadmium cells, lead-acid cells, and lithium cells. First, the hydrogen storage secondary cells contain neither cadmium nor lead nor lithium; as such they do not present a consumer safety or environmental hazard. Second, electrochemical cells with hydrogen storage negative electrodes offer significantly higher specific charge capacities than do cells with lead or cadmium negative electrodes. As a result, a higher energy density is possible with hydrogen storage cells than with conventional systems, making hydrogen storage cells particularly suitable for many commercial applications.
A number of different hydrogen alloy systems have been proposed for use in nickel-metal hydride hydrogen storage electrochemical cells. One such system is the AB.sub.2 -type hydrogen storage alloy characterized by C.sub.14 and C.sub.15 type Laves phase crystalline microstructures. Materials of this type are further characterized by large grain sizes occurring naturally under conventional solidification processes. These materials typically comprise one or more of titanium, zirconium and hafnium and nickel plus one or more additional metals. It is interesting to note that the earliest teaching of AB.sub.2 -type hydrogen storage alloy materials are, as is the case for many of hydrogen storage systems, directed to thermal hydrogen storage alloys. In thermal hydrogen storage alloys, driving forces for hydriding and dehydriding are thermal and pressure the driving forces. By way of contrast, electrochemical hydrogen storage alloys are charged and discharged by an electron transfer process in ionic media.
A number of different practitioners in the hydrogen storage alloy material art have published extensive amounts of literature in the area of AB.sub.2 -type hydrogen storage alloy materials. While the bulk of this work is directed to thermal hydrogen storage alloys, extensive teaching of prior art Laves phase type electrochemical hydrogen storage alloys are shown, for example, in Matsushita Electric Industrial Company, Ltd. laid-open European patent application 0-293 660 based on European patent application 88107839.8, filed May 16, 1988 and claiming priority to Japanese patent applications 87/119411, 87/190698, 87/205683, 87/21698, and 87/258889. The teaching of these applications is collectively presented in Japanese patent 89-102855 issued Apr. 20, 1989 to Moriwaki, et al entitled "HYDROGEN STORAGE ALLOY ELECTRODE" issued on Japanese patent application 87JP-258889, filed Oct. 14, 1987. This patent discloses multidimensional hydrogen storage alloys and their hydrides. The alloys are disclosed to be C.sub.15, Laves phase type materials, which materials have the chemical formula expressed by A.sub.x B.sub.y Ni.sub.z where A is zirconium either alone or with one or more of titanium and hafnium, the titanium or hafnium being 30 atomic percent or less; x equals 1.0; B is at least one of the elements niobium, chromium, molybdenum, manganese, iron, cobalt, copper, aluminum and rare earths elements such as lanthanum and cerium; y is between 0.5 and 1.0; z is between 1.0 and 1.5 and the sum of y plus z equals 1.5 to 2.5.
Moriwaki, et al disclose that this composition enhances the hydrogen storing ability of the alloy and suppresses the loss of discharge capacity which occurs after repeated charging-discharge cycling (cycle life) of titanium-nickel and zirconium-nickel binary systems.
The teaching of the C.sub.14 and C.sub.15 type materials is noteworthy because processing conditions are frequently specified to include standard cooling rates and even annealing steps so as to produce alloys having stable microstructures. Additional AB.sub.2 type materials are disclosed in, for example, Japanese patent numbers 63-284758, 89-035863, 89-048370, 89-060961. It is important to note that the teaching of each of these cases is deficient with regard to the invention disclosed hereinbelow and that one or more of the key elements taught herein is missing, for example, a teaching which is deficient in each of said above references is that of preferred microstructure and optimum grain size. Further, it is important to note that each of said references is selective or limited to very specific compound which compounds do not disclose, teach or suggest the hydrogen storage alloy material taught herein.
Another suitable class of electrochemical hydrogen storage alloys are the titanium, vanadium, zirconium, and nickel (Ti-V-Zr-Ni) type active materials for the negative electrodes of electrochemical cells. These materials are disclosed in, for example, commonly assigned U.S. Pat. Nos. 4,551,400 to Sapru, Hong, Fetcenko and Venkatesan and 4,728,586 to Venkatesan, Reichman and Fetcenko, the disclosures of which are incorporated herein by reference. The '586 patent of Venkatesan, et al, entitled "ENHANCED CHARGE RETENTION ELECTROCHEMICAL HYDROGEN STORAGE ALLOYS AND AN ENHANCED CHARGE RETENTION ELECTROCHEMICAL CELL" describes a specific sub-class of titanium, vanadium, nickel, zirconium, hydrogen storage alloys comprising titanium, vanadium, zirconium, nickel and a fifth component, particular by chromium. In the preferred exemplification of Venkatesan, et al, the hydrogen storage alloy material has the composition: EQU Ti.sub.0.33-x Zr.sub.x V.sub.0.67-y Ni.sub.y).sub.1-z Cr.sub.z
where x is from 0.0 to 0.25, y is from 0.1 to 0.6 and z is an effective amount between 0.00 and 0.20.
It is important to note that in the '586 patent, the two critical parameters taught hereinbelow are lacking. More specifically, in the '586 patent the average grain size of the material therein is on the order of approximately 10 to 30 microns. Additionally, perhaps more importantly, is the presence of at least one phase in the multiphase, multi-component material which comprises greater than 50 percent vanadium and chromium combined. Indeed, in most cases this heavy V-Cr phase comprises greater than 70 percent vanadium and chromium. The instant inventors have found that multiphase, multi-component materials having larger grain sizes and a heavy vanadium-chromium phase, while displaying excellent charge retention characteristics, also display diminished cycle life capabilities. It is to be particularly noted that the disclosure of the '586 patent specifically recites the presence of the heavy V-Cr phase, and large grain sizes which distinguish it from the teaching of the instant invention.
It is a feature of the instant invention that the hydrogen storage alloy materials, in order to possess a requisite small grain size and reduce the amount or substantially eliminate the presence of the heavy vanadium-chromium phase, be fabricated by rapid solidification from a melt.
The use of fabrication techniques wherein a hydrogen storage alloy material are made by rapid solidification from the melt is disclosed in commonly assigned U.S. Pat. No. 4,637,967 to Keem, et al for "ELECTRODES MADE WITH DISORDERED ACTIVE MATERIAL AND METHODS OF MAKING THE SAME" the disclosure of which is incorporated herein by reference. Keem, et al teach the fabrication of a self supporting, non-particulate dimensionally anisotropic, amorphous negative electrode for use in an electrochemical cell comprising at least titanium, nickel and one element selected from the group of aluminum, boron, chromium, cobalt, hafnium, indium, lead, magnesium, molybdenum, niobium, palladium, tin, zirconium, tin, zirconium and rare earth metals is fabricated. As used by Keem, et al self supporting means a ribbon of electrode fabricated by the direct rapid solidification from a melt of hydrogen storage alloy material, which ribbon is directly employed as the negative electrode without subsequent processing steps. Thus, the Keem, et al reference while pertinent generally to the field of rapid solidification from a melt in hydrogen storage materials, fails to teach a five component alloy including Ti-Zr-V-Ni-Cr and further is limited to a self supporting, amorphous structure. As is well-known in the rapid solidification art, materials formed by rapid solidification are typically substantially non porous. Given that electrochemical reactions, such as those which take place in an electrochemical cell, are generally surface reactions, i.e., heavily dependent upon the surface area and surface character of the electrodes, such non-porous, non-particulate electrodes such as those taught by Keem, et al. are in fact undesirable in many electrochemical applications. Further, given a continuous amorphous structure throughout, the surface area and surface conditions of said electrodes are quite poor for purposes of electron transfer in an electrochemical cell. Therefore, the teaching of the Keem, et al reference, in fact, leads one of ordinary skill in the battery art away from employing the rapid solidification from a melt since the resulting electrode typically results in a electrode ribbon possessing characteristics which are undesirable for electrochemical reactions.
Accordingly, it can be seen that there is a need for a particulate, porous, multi-component, multiphase hydrogen storage alloy material fabricated so said multiphase material does not possess heavy amounts of the predominantly vanadium chromium phase and further so that said material is characterized by relatively small grain sizes therein.